Anti-Invar in Fe-Ni

Abstract

Anti‐Invar behavior in a material can be characterized by an anomalously large and a weak temperature‐dependent thermal‐expansion coefficient, when compared to the respective Grüneisen lattice expansion. It is just the opposite of the Invar effect, which is characterized by an anomalously small thermal‐expansion coefficient. Common to Invar and anti‐Invar is the fact that both posess moment‐volume instabilities. Anti‐Invar is observed in the paramagnetic state. It occurs in γ‐Fe and in a number of 3d fcc binary and ternary alloys. In Fe_xNi_100−x alloys it is observed in the concentration range 70≤x≤100 at. % within the fcc stability range. The effect vanishes as the Invar concentration, x=65 at. %, is approached. To examine the valence electron concentration dependence of the anti‐Invar effect the thermal expansion has been measured in the fcc state of Fe_xNi_100−x for 63≤x≤100 at. %. Using a model based on moment‐volume instabilities in conjunction with a thermal activation process the size of the anti‐Invar effect in these alloys has been determined. The volume enhancement is found to decrease with increasing Ni concentration from 2.8% in γ‐Fe to 0% at x=65 at. %.